Generally, in an image tube such as CRT, the relationship between drive voltage E and cathode current I is expressed by the following formula.I=KEγγ:gamma
This relationship is referred to as the gamma (γ) characteristic. Cathode current I has non-linearity in response to drive voltage E since the value of gamma is generally around 1.6 to 3. An image transmission side totally sets γ=1 by installing a gamma-correction circuit since it is difficult to set γ=1 in an image tube. In the present application, a gamma conversion is defined as implementing conversion to an image signal as gamma correction.
In an image reading device or an image processing LSI for an image display unit, image data is gamma-converted. When an input is x and an output is y, gamma conversion is expressed as the following formula.
                    Equation        ⁢                                  ⁢        1                                                            y        =                  x                      1            γ                                              (        1        )            
FIG. 26 shows a chart of formula (1) when gamma γ=1.8. Gamma conversion is a power function so that a lot of time is necessary to calculate it. However, high-speed gamma-conversion is required since image-reading equipment is operated with high speed. Therefore, in a conventional technique, a conversion table of converted data corresponding to input data is stored by a high-speed memory cell such as SRAM. Then, converted data is read out from a conversion table corresponding to input data.
FIG. 27 shows a diagram of a conventional gamma-conversion circuit. As shown in FIG. 27, a conventional gamma-conversion circuit 70 is provided with a conversion-table storing portion 71 to store a conversion table of converted data corresponding to input data. The circuit 70 outputs the converted data corresponding to the input data. In this case, the conversion-table storing portion 71 is in the form of a high-speed memory cell SRAM.
In gamma-conversion circuit 70, when the input data is 12 bits, and the output data is 8 bits, for example, the size of the conversion table becomes 4096 words (here, a single word=8 bits) so that it is necessary for the memory capacity of the conversion-table storing portion 71 to hold 4096 words. Further, when the input data is 16 bits, and the output data is 12 bits, the size of the conversion table becomes 65536 words (here, a single word 12 bits) so that it is necessary for the memory capacity of the conversion-table storing portion 71 to hold 65536 words. It is extremely difficult to integrate SRAM of a high capacity for storing such large sized conversion tables into a LSI for image processing. Further, even if a SRAM of such a high capacity can be integrated into a LSI for image processing, this causes cost increase of the LSI for image processing.
Further, in order to make the conversion table small, the conversion table having the converted data corresponding to the input data every predetermined period (for example, 16) is stored in a SRAM. When the input data corresponding to the converted data is stored in the conversion table is input, such converted data is output. When the input data which does not correspond to the converted data stored in the conversion table is input, other input data located before or after the subject input data, which corresponds to the converted data stored in the conversion table, is processed with interpolation and output.
FIG. 28 shows a diagram of such a conventional gamma-conversion circuit. As shown in FIG. 28, a conventional gamma-conversion circuit 80 comprises a conversion-table storing portion 81, which stores the conversion table having the converted data corresponding to the input data every specific period and a processing portion for interpolation 82 to process interpolation on the output data from the conversion-table storing portion 81. In this case, the conversion-table storing portion 81 is in the form of high-speed memory cells of SRAM.
In the gamma-conversion circuit 80, when the input data is 12 bits, the output data is 8 bits and a period of the input data corresponding to the converted data is 16, for example, the size of the conversion table becomes 256 words (here, a single word=8 bits) and the memory capacity needed for the conversion-table storing portion 81 is reduced to 256 words. Further, when the input data is 16 bits, the output data is 12 bits and a period of the input data is 16, the size of the conversion table becomes 4096 words (here, a single word=12 bits) and the memory capacity needed for the conversion-table storing portion 81 is reduced to 4096 words.
However, the output data from the gamma-conversion circuit 80 includes errors since these are calculated by interpolation processing. FIG. 29 shows the errors between the formula (1) and output data from the gamma-conversion circuit 80 when the input data is 16 bits, the output data is 16-bits, and a period of the input data corresponding the converted data is 16. As shown in FIG. 29, the output data from the gamma-conversion circuit 80 has about 130 errors at its maximum.
Therefore, in view of the foregoing, one object of the present invention is to decrease memory capacity of the circuit for storing converted data and to provide a data conversion method, a data conversion circuit and a data conversion program while decreasing the errors in the output data.